Method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention

ABSTRACT

Disclosed is a method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention, belonging to the technical field of natural gas hydrates mining. A grouting layer is constructed between a natural gas hydrates reservoir layer and an upper overburden layer by adopting a multi-branch horizontal well process, a vertical shaft is drilled further into the natural gas hydrates reservoir layer after the grouting layer is stabilized, and a mining device and a bottom plugging device are installed in the vertical shaft according to a thickness of the reservoir.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.2022101859356, filed on Feb. 28, 2022, the contents of which are herebyincorporated by reference.

TECHNICAL FIELD

The present application relates to a deep-sea mining method of hydrates,and in particular to a method for deep-sea extraction of natural gashydrates with reservoir top control and sand prevention.

BACKGROUND

As one of the potential energy sources with large amount of reserves andhigh energy density, natural gas hydrates are cage crystals formed bynatural gas and water under high pressure and low temperatures, widelydistributed in deep-water strata such as high latitude polar tundra,oceans and lakes. Among the components of natural gas, methane has ahigh energy density (volume of methane per unit of rock volume atstandard conditions), which is 10 times that of coal and black shale and2.5 times that of natural gas.

Areas where natural gas hydrates are widely distributed include theslopes of continents and islands, uplifts on the edges of active andpassive continental margins, and the deep-water environments of polarcontinental shelves, oceans and some inland lakes; and the conditionsfor the formation of natural gas hydrates include: low temperature,generally below 10 degrees Celsius (° C.); high pressure, generallyabove 10 megapascals (MPa); sufficient natural gas sources(hydrocarbons, mainly methane); and favorable hydrate occurrence space.

The rock matrix of the hydrate reservoir layer is usuallyunconsolidated, weakly consolidated or fractured layers, which aresusceptible to changes in the cementation strength, porosity, effectivestress and permeability coefficient of the layer as a result of hydrateextraction; moreover, as hydrate is layered in the subsea strata,phenomena of water and sand production from the layers are prone tooccur due to the difference in permeability between the hydrate layerand the overburden layer during the mining process, in addition to thedisturbance of the original seepage equilibrium caused by hydratedecomposition; as a result, the stability of the well wall isdeteriorated, leading to well collapse, which seriously affects theeffective exploitation of hydrate resources. Thus, layer deformation andsand control become the key issues in the hydrate decomposition process,and an effective mining method that can control subsea layer deformationwhile slowing down sand emergence from the well is urgently needed.

SUMMARY

One of the objectives of the present application is to provide a methodfor deep-sea extraction of natural gas hydrates with reservoir topcontrol and sand prevention, so as to solve the problems of layerdeformation and sand production in the process of mining natural gashydrates in marine areas, and to improve the mining of deep-sea naturalgas hydrates with safety and efficiency.

In order to achieve the above objectives, the present applicationprovides a technical scheme as follows:

a method for deep-sea extraction of natural gas hydrates with reservoirtop control and sand prevention, including:

S1, determining an area where natural gas hydrates are located, andanalyzing the area for geological parameters, including stratumpermeability coefficient, stratum temperature and grain composition;

S2, determining a drilling position in the area and setting up anoffshore production platform; and constructing a vertical shaft on theoffshore production platform to penetrate an overburden layer and extendto a boundary position between the overburden layer and a natural gashydrates reservoir layer;

S3, around the vertical shaft, arranging and opening horizontal wells atthe boundary position between the overburden layer and the natural gashydrates reservoir layer with the vertical shaft as a center;

S4, grouting slurry in a horizontal direction of the horizontal wells toform a grouting layer;

S5, installing a plugging device at a junction of the horizontal wellsand the vertical shaft after the slurry is solidified;

S6, extending the vertical shaft to the natural gas hydrates reservoirlayer in the area, and installing a mining device and a drilling holebottom plugging device on the vertical shaft; and

S7, mining the natural gas hydrates.

The overburden layer in the S2 is below a marine layer; the verticalshaft penetrates the overburden layer and is located at a boundarybetween the natural gas hydrates reservoir layer and the overburdenlayer.

The vertical shaft in the S3 is located by its bottom in the boundarybetween the natural gas hydrates reservoir layer and the overburdenlayer, and the horizontal wells are constructed in the boundary betweenthe natural gas hydrates reservoir layer and the overburden layer.

The grouting in the horizontal wells in the S4 includes determining agrouting method according to spacings and bearing capacities of thehorizontal wells 5, and the grouting method includes compactiongrouting, split grouting, and combining grouting of split grouting andcompaction grouting.

The grouting is carried out at a pressure of 3-5 times that of a porewater pressure in the horizontal wells.

The slurry in the grouting layer is water-based liquid retardingmaterials.

The plugging device is used for sealing the horizontal wells at theirwellheads.

The present application has the following technical effects: a groutinglayer is constructed between the natural gas hydrates reservoir layerand the upper overburden layer by adopting a multi-branch horizontalwell process, so that the permeability of the upper part of the naturalgas hydrates reservoir layer is reduced, the movement of water from theupper overburden layer to the natural gas hydrates reservoir layer isreduced, and the upper part of the natural gas hydrates is thereforeimproved in terms of mechanical strength, with mitigated deformation andsubsidence of subsea stratum and sand emergence from the wellbore causedby extracting natural gas hydrates. The present application has greatvalue for promotion and application in the technical field of deep-seamining; it is designed with a clear concept and a simple and operableconstruction method, so as to effectively improve the stability of thesubsea formation during the extraction of deep-sea natural gas hydratesand reduce the accidents of sand production from the wellbore caused bythe water produced during extraction.

BRIEF DESCRIPTION OF THE DRAWINGS

For a clearer illustration of the technical schemes in the embodimentsof the present application or in the prior art, a brief description ofthe accompanying drawings to be used in the embodiments is providedbelow. It is apparent that the accompanying drawings in the followingdescription are only some embodiments of the present application andthat other drawings are available on the basis of these drawings to aperson of ordinary skill in the field without any effort of creativelabor.

FIG. 1 shows a schematic diagram of horizontal wells construction of thepresent application.

FIG. 2 is a schematic diagram of grouting slurry in the horizontal wellsaccording to the present application.

FIG. 3 shows a schematic plan view of grouting slurry in the horizontalsaccording to the present application.

FIG. 4 is a schematic diagram illustrating hydrates exploitation of thepresent application.

FIG. 5 illustrates a schematic diagram of seabed stratum subsidencewithout grouting layer.

FIG. 6 illustrates a schematic diagram of seabed stratum subsidenceafter being reinforced by the grouting layer of the present application.

FIG.7 is a process illustrating a method for deep-sea extraction ofnatural gas hydrates with reservoir top control and sand preventionaccording to one embodiment of the present application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The technical schemes in the embodiments of the present application aredescribed clearly and comprehensively below in conjunction with theaccompanying drawings in the embodiments of the present application. Itis clear that the embodiments described are only a part of theembodiments of the present application and not all of them. Based on theembodiments in the present application, all other embodiments obtainedwithout creative labor by a person of ordinary skill in the art fallwithin the scope of protection of the present application.

In order to make the above-mentioned objectives, features and advantagesof the present application more obvious and understandable, the presentapplication is described in further detail below in conjunction with theaccompanying drawings and specific embodiments.

The present application provides a method for deep-sea extraction ofnatural gas hydrates with reservoir top control and sand prevention asshown in FIG. 7 , including:

S1, determining an area where natural gas hydrates are located, andanalyzing the area for geological parameters, including permeabilitycoefficient of natural gas hydrates stratum, and grain composition;

S2, determining a drilling position in the area and setting up anoffshore production platform 1; and constructing a vertical shaft 3 onthe offshore production platform 1 to penetrate an overburden layer 4and extend into the area;

S3, arranging and opening horizontal wells 5 in the area with thevertical shaft 3 as a center;

S4, grouting slurry in a horizontal direction of the horizontal wells 5to form a grouting layer 8;

S5, installing a plugging device 9 at a junction of the horizontal wells5 and the vertical shaft 3 after the slurry is solidified;

S6, extending the vertical shaft 3 to a natural gas hydrates reservoirlayer 6 in the area, and installing a mining device 10 and a drillinghole bottom plugging device 11 on the vertical shaft 3; and

S7, mining the natural gas hydrates.

The overburden layer 4 in the S2 is below a marine layer 2; the verticalshaft 3 penetrates through the overburden layer 4 and is located at aboundary between the natural gas hydrates reservoir layer 6 and theoverburden layer 4.

The vertical shaft 3 in the S3 is located by its bottom in the boundarybetween the natural gas hydrates reservoir layer 6 and the overburdenlayer 4, and the horizontal wells 5 are constructed in the boundarybetween the natural gas hydrates reservoir layer 6 and the overburdenlayer 4.

The grouting in the horizontal wells 5 in the S4 includes determining agrouting method according to spacings and bearing capacities of thehorizontal wells 5, where the grouting method includes compactiongrouting, split grouting, and combining grouting of split grouting andcompaction grouting.

The grouting is carried out at a pressure of 3-5 times that of a porewater pressure in the horizontal wells 5.

The slurry in the grouting layer 8 is water-based liquid retardingmaterials.

In one embodiment of the present application, the slurry is injectedinto the boundary between the overburden layer 4 and the natural gashydrates reservoir layer 6, so as to fill up pore particles in theboundary and displace the seawater therein and solidify and cement thesurrounding particles.

The plugging device 9 is used for sealing the horizontal wells 5 attheir wellheads.

In one embodiment of the present application, the area where natural gashydrates are located is generally found below a subsea sediment layer,with the overburden layer 4 of seawater mixed with particulate poresbetween the marine layer 2 and the natural gas hydrates reservoir layer6, and a deeper lower layer 7 at the bottom of the natural gas hydratesreservoir layer 6;

according to the present application, as shown in FIG. 1 to FIG. 3 , thevertical shaft 3 is firstly constructed to a designed horizon located inthe boundary between the overburden layer 4 and the natural gas hydratesreservoir layer 6, and the grouting layer 8 is formed in the horizontalwells 5 by using the stratum grouting process, where the grouting layer8 of a plate structure is designed to lower the permeability at the topof the natural gas hydrates reservoir layer 6, and reduce the flow ofwater in the overburden layer 4 to the natural gas hydrates reservoirlayer 6, so as to improve the mechanical strength of the upper part ofthe natural gas hydrates and mitigate the deformation and subsidence ofthe seabed stratum caused by natural gas hydrates mining.

In the prior art, there is generally no protective setup in the processof extraction, with only one vertical shaft 3 opened directly into thenatural gas hydrates reservoir layer 6; or, the horizontal wells 5 arearranged and the extraction is performed directly downwards; ascomparing to the technical schemes of the present application, theseawater and sand in the overburden layer 4 applied with prior art areprone to subside, thus affecting the mining device and causingaccidents.

In a further optimized technical scheme, the vertical shaft 3 isdesigned to be constructed in sections, which is different from avertical shaft 3 directly connected to the natural gas hydratesreservoir layer 6 in the prior art. In the present application, it ispossible to construct two vertical shafts 3 at the same position, withone being constructed to the designed horizon to build the groutinglayer; while the other vertical shaft 3 is extended to the natural gashydrates reservoir layer 6 by a same construction mode, then sealed bythe drilling hole bottom plugging device 11, and the natural gashydrates are mined by the mining device.

In a further optimized technical scheme, the pressure of grouting andthe spacings between the horizontal wells 5 are designed and determinedaccording to the geological parameters including permeabilitycoefficient of natural gas hydrates stratum, stratum temperature andgrain composition in S1, and the specific grouting method is determinedby a ratio of the grouting pressure to the pore water pressure inhorizontal wells 5.

Embodiment 1

Deep-sea drilling data is utilized to analyze the geological parameterssuch as stratum permeability coefficient, stratum temperature and graincomposition in the area where natural gas hydrates are located, and theposition of vertical shaft 3 and the thickness of overburden layer 4 aredetermined, then the stratum pressure of regional strata is calculatedwith reference to the depth of marine layer; then the location of theboundary between overburden layer 4 and natural gas hydrates reservoirlayer 6 is determined according to logging curves and geological radardata, and the diameter and spacings of horizontal wells 5 are determinedaccording to the mining area; taking the occurrence of natural gashydrates reservoir layer in a certain sea area as an example, thespecific data are as follows: seawater depth of 800 meters (m),thickness of overburden layer of 200 m, stratum permeability coefficientof 1.2×10⁻⁶ centimeters per second (cm/s), stratum pressure of 12megapascals (A/Pa), stratum temperature of 4 degree Celsius (° C.), andaverage particle size of stratum particles of 100 micrometers (μm),diameter of the vertical shaft of 2 m, diameter of the horizontal well 5of 0.3 m, where 6 horizontal wells at 60° intervals centered on thevertical shaft 3 are arranged with a drilling depth of 10 m;

the offshore production platform 1 and the vertical shaft are thenconstructed on the basis of the above parameters, where the verticalshaft 3 penetrates through the overburden layer 4 to the designedposition of the horizontal wells 5; considering the extremely fineaverage particle size of the stratum particles, the method of splitgrouting is adopted to grout slurry in the horizontal wells 5, with agrouting pressure of 40 MPa; then the grouting layer 8 is developed andthe plugging device 9 is installed to separate the horizontal wells 5from the passage of the vertical shaft 3; then the construction isextended to the natural gas hydrates reservoir layer 6;

Further, the natural gas hydrates are mined by the mining device 10 inthe vertical shaft 3, as shown in FIG. 4 .

Further, in the present embodiment, the grouting layer 8 is formed tostably isolate the overburden layer 4 form the natural gas hydratesreservoir layer 6; the permeability at the top of the natural gashydrates reservoir layer 6 and the flow of water in the overburden layer4 to the natural gas hydrates reservoir layer 6 are reduced accordingly,and the mechanical strength of the upper part of the gas hydrates isimproved so as to mitigate the deformation and subsidence of the seabedstratum caused by the gas hydrates exploitation, see FIG. 5 and FIG. 6for comparison.

Embodiment 2

The present embodiment is different form Embodiment 1 in that thegrouting method of compaction grouting, or combining grouting of splitgrouting and compaction grouting is adopted when the stratumpermeability coefficient is larger than 3×10⁻⁵ cm/s, and the averageparticle size of stratum particles is greater than 1 millimeter (mm) inthe horizontal wells 5.

In the description of the present application, it is to be understoodthat the terms “longitudinal”, “transverse”, “up”, “down”, “front”,“back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”,“inside”, “outside”, etc. indicate an orientation or positionalrelationship based on the orientation or positional relationship shownin the accompanying drawings and are intended only to facilitate thedescription of the invention and do not indicate or imply that thedevice or element referred to must have a particular orientation, or beconstructed and operated in a particular orientation, and are thereforenot to be construed as limiting the present application.

The above-mentioned embodiments only describe the preferred mode of thepresent application, and do not limit the scope of the presentapplication. Under the premise of not departing from the design spiritof the present application, various modifications and improvements madeby ordinary technicians in the field to the technical schemes of thepresent application shall fall within the protection scope determined bythe claims of the present application.

What is claimed is:
 1. A method for deep-sea extraction of natural gashydrates with reservoir top control and sand prevention, comprising: S1,determining an area where natural gas hydrates are located, andanalyzing the area for geological parameters, comprising a stratumpermeability coefficient, a stratum temperature and grain composition;S2, determining a drilling position in the area and setting up anoffshore production platform; and constructing a vertical shaft on theoffshore production platform to penetrate an overburden layer and extendto a boundary position between the overburden layer and a natural gashydrates reservoir layer; S3, around the vertical shaft, arranging andopening horizontal wells at the boundary position between the overburdenlayer and the natural gas hydrates reservoir layer with the verticalshaft as a center; S4, grouting slurry in a horizontal direction of thehorizontal wells to form a grouting layer; S5, installing a pluggingdevice at a junction of the horizontal wells and the vertical shaftafter the slurry is solidified; S6, extending the vertical shaft to thenatural gas hydrates reservoir layer in the area, and installing amining device and a drilling hole bottom plugging device on the verticalshaft; and S7, mining the natural gas hydrates.
 2. The method fordeep-sea extraction of natural gas hydrates with reservoir top controland sand prevention according to claim 1, wherein the overburden layerin the S2 is below a marine layer; the vertical shaft penetrates throughthe overburden layer and is located at a boundary between the naturalgas hydrates reservoir layer and the overburden layer.
 3. The method fordeep-sea extraction of natural gas hydrates with reservoir top controland sand prevention according to claim 2, wherein the vertical shaft inthe S3 is located by a bottom in the boundary between the natural gashydrates reservoir layer and the overburden layer, and the horizontalwells are constructed in the boundary between the natural gas hydratesreservoir layer and the overburden layer.
 4. The method for deep-seaextraction of natural gas hydrates with reservoir top control and sandprevention according to claim 1, wherein the grouting in the horizontalwells in the S4 comprises determining a grouting method according tospacings and bearing capacities of the horizontal wells, where thegrouting method comprises compaction grouting, split grouting, andcombining grouting of the split grouting and the compaction grouting. 5.The method for deep-sea extraction of natural gas hydrates withreservoir top control and sand prevention according to claim 4, whereinthe grouting is carried out at a pressure of 3-5 times of a pore waterpressure in the horizontal wells.
 6. The method for deep-sea extractionof natural gas hydrates with reservoir top control and sand preventionaccording to claim 1, wherein the slurry in the grouting layer iswater-based liquid retarding materials.
 7. The method for deep-seaextraction of natural gas hydrates with reservoir top control and sandprevention according to claim 1, wherein the plugging device is used forsealing the horizontal wells at wellheads.